1. Field of the Invention
The invention is concerned with the packaging of optical fibers, especially optical sensor fibers, both for shipment and in a form convenient for use.
2. Description of the Related Art
As is pointed out in U.S. Pat. No. 4,432,607 (Levy):
"Optical fibers require protective coatings in order to preserve fiber strength and to protect the fiber from microbending induced optical loss. The coating is generally applied in-line with fiber drawing by passing the fiber through a reservoir containing the coating material and having an exit orifice which has been sized to apply some desired thickness of the material . . . Classes of materials which have been applied to optical fibers by this technique include ultraviolet (UV) curables, thermal curables, solvent-based materials, and hot melts" (col. 1, lines 9-22).
The Levy patent discusses problems of achieving a coating of sufficient toughness, abrasion resistance, adhesion to the fiber, and thickness within the time and space available while also realizing a useful lifetime in the reservior. These problems are said to be eliminated by a hot-melt optical fiber coating material based on a block copolymer.
U.S. Pat. No. 4,642,265 (Suzuki) says that optical communication glass fibers are normally protected by a resin of relatively low Young's modulus overcoated with a resin of relatively high Young's modulus, but that the inner coating often had been difficult to remove to make connections. The Suzuki patent answers this problem with a special silicone inner coating covered with a thermoplastic resin material.
U.S. Pat. No. 4,351,657 (Kimura et al.) teaches a special method of applying a coating of thermoplastic resin to an optical fiber in-line with the drawing of the fiber, which method can be used at very high line speeds. In order to improve the tensile strength of the optical fiber, a silane coupling agent may be applied ahead of the thermoplastic coating.
Protectively coated optical fibers like those of the above-discussed patents typically are level-wound on spools for storage and shipment, usually under considerable tension. When an optical fiber is to be used while on a spool, e.g., in a gyro, it is wound loosely to avoid attenuation due to tension. While being loosely wound, the fiber is often coated with a resin that bonds adjacent convolutions together by curing quickly upon exposure to ultraviolet radiation, thus preventing the fiber from cascading or becoming entangled when the spool is handled. The resulting coils are self-supporting and sometimes have been supplied on collapsible spools to permit either end of a coil to be unwound and fit into connectors or otherwise connected into an opto-electronic system. The ultraviolet curing required the coil to be wound at very low speeds, and unwinding also was necessarily slow to avoid damaging the optical fiber.